This proposal concerns the further characterization of the two bactericidal/permeability increasing proteins (BPI), one from human and one from rabbit polymorphonuclear leukocytes (PMN), that have been purified previously by this group. The isolated proteins are potently bactericidal toward a range of gram-negative bacterial species, but are non-toxic to gram- positive bacteria or eukaryotic cells. In intact PMN BPI appears to be the major 02-independent bactericidal agent against sensitive bacterial species. Our specific aims are: I. To relate structure and function in the human and rabbit BPI by protein-analytical, immunological and recombinant DNA techniques. II. To examine further how the interaction of BPI with the gram-negative bacterial envelope relates to effects on viability and envelope structure and function. III. To explore further the biological importance of BPI in host- defense against gram-negative bacterial infections, using intact phagocytes, anti-BPI antibodies, nonbactericidal binding fragments of BPI and transfection of the BPI-gene into phagocytes that do not express this gene. The pursuit of these aims will rely on our previous work and the use of well-tested methods, including protein purification and radiolabeling, assays for BPI action, bacterial mutants, immunological methods, cells in culture. Our long-term objectives are to understand better leukocyte function and host-defense against infection, and should be met by the proposed studies on these two well-defined proteins. The planned scrutiny of structure-function relationships in BPI may help the design of new antibiotics.